Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers, and the power amplifier may often be the primary consumer of power in a personal audio device, and thus, may have the greatest effect on the battery life of the personal audio device. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, because the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Therefore, amplifier topologies such as Class-G and Class-H are desirable for reducing the voltage drop across the output transistor(s) and thereby reducing the power wasted in dissipation by the output transistor(s).
In order to provide a variable power supply voltage to such a power amplifier, a charge pump power supply may be used, for example such as that disclosed in U.S. Pat. No. 8,311,243, in which an indication of the signal level at the output of the circuit is used to control the power supply voltage in a Class-G topology. The above-described topology may raise the efficiency of the audio amplifier, in general, as long as periods of low signal level are present in the audio source. Typically in such topologies, a plurality of thresholds define output signal level-dependent operating modes for the charge pump power supply, wherein a different supply voltage is generated by the charge pump power supply in each mode.
In a typical charge pump power supply, a charge pump may operate in accordance with two non-overlapping clock phases of a switching cycle of the charge pump, with different combinations of connections among an input power source to the charge pump (e.g., a battery), a flying capacitor of the charge pump for storing charge, and an output load capacitor which provides the power supply voltage generated by the charge pump. However, one disadvantage of a charge pump may occur when switching between output voltage modes of the charge pump. In general, the voltage on the flying capacitor of the charge pump may be equal to the voltage of the input power source. If the output voltage is smaller than the sum of the input power source voltage and the flying capacitor voltage during an increase in charge pump ratio, the charge pump may need to source a large inrush current from its power source (e.g., a battery). On the other hand, if the output voltage is greater than the sum of the input power source voltage and the flying capacitor voltage, the charge pump may need to sink large current to its power source. Because of the sizes of capacitors often used in charge pumps, the amount of current that a charge pump may source or sink when switching between modes may not be able to be absorbed by the power source to the charge pump, which may lead to system damage. Accordingly, methods and systems for limiting such switching currents are desirable.
Examples of systems and methods for current limiting in amplifiers with boosted charge pump power supplies, and conditions for initiating such current limiting, are described in U.S. application Ser. No. 15/783,506 filed Oct. 13, 2017, U.S. application Ser. No. 15/800,743 filed Nov. 1, 2017, and U.S. application Ser. No. 15/823,140 filed Nov. 27, 2017, all of which are incorporated by reference herein.